EA: numeral + ت-initial plural

When I first started studying Egyptian Arabic, I learned that أَيّام becomes تِيّام after a numeral, أُشهُر becomes تُشهُر, and أَلاف becomes تَلاف. (It's curious that the initial vowel only changes with أَيّام.) I'm reviewing grammar through Abdel-Massih's EA textbooks from the '70s, and he reports that the same is true of the plurals of سطر and خُمس, and optionally قلم. Is there a name for this phenomenon in Arabic?

Perhaps I've explained this poorly: The point isn't that the مضاف of خمسَة is خمسِة, but rather that the the plural of خُمس, which is would otherwise be أخماس, is pronounced تخماس after a numeral. So, for example, there's the idiom ضرب أخماس فى أسداس, where you've got the sort of normal plural, but that the fraction 4/5 is pronounced أربع تَخماس.

For what it's worth, I'm not saying this is so on that specific word: I don't know that I've ever heard either the word خمس or أخماس in speech. For خمس and سطر and قلم, I'm just reporting what the book says. But I do think I've heard people say ثمن تِيّام, which the book is referring to as the same phenomenon.

Edit: Oh, I see. My apologies: I thought that the response concerning the مضاف of خمسة was in reply to the bit on the plural of خُمس, rather than on the specific example from إسكندرانى. That makes sense to me. But then wouldn't this be something to note about the pronunciation of the مضاف of the numeras with all plurals in افعال? Is there really nothing different about أيّام and أشهر?

Thanks much.

Edit: Apologies again: I just went digging into another grammar (Kullu Tamām), and found a claim that it does apply to all أفعال plurals. So that was a poor explanation!